1. Field of the Invention
The present invention concerns power thyristors having a high triggering speed.
2. Description of the Prior Art
It is known that these thyristors comprise:
A semi-conductor monocrystalline plate with an anode face and a cathode face, that plate comprising successively, from the anode face to the cathode face, an anode of the P type, an intermediate layer of the N type, a base of the P type and a layer of emitters of the N type having gaps taken up by the base, so that the cathode face comprises, at least, a central region of the base, a triggering emitter surrounding that central region, an intermediate region of the base surrounding that triggering emitter and a main emitter surrounding that intermediate region and having a greater area than the said triggering emitter;
A trigger formed by a metallic layer deposited on the said central region of the base at a distance from the said triggering emitter;
An "auxiliary" metal spray deposited on the said triggering emitter remaining at a distance from the said central region of the base and overlapping onto the said intermediate region of the base;
A cathode formed by a metallic layer deposited on the said main emitter at a distance from the said intermediate region of the base;
Two main terminals connected, the one to the said anode and the other to the said cathode;
A control terminal connected to the said trigger;
The function of the triggering emitter is also known: It is intended to make the firing easier by means of a low-power pulse:
The "triggering" emitter is distinct from the "main" emitter and has a smaller area. It is arranged between the trigger and the main emitter so that the thyristor being extinguished and under direct voltage between the anode and the cathode situated on the main emitter, the potential of the triggering emitter being floating, a suitable positive signal applied to the trigger fires firstly a triggering thyristor whose emitter is constituted by the triggering emitter.
To make the description more understandable, various more or less successive phases in the firing process may, very roughly, be distinguished:
First phase:
The signal applied to the trigger causes the injecting of minority carriers in the base from the triggering emitter.
Second phase:
The above injection fires the triggering thyristor according to the conventional process, this raising the potential of the triggering emitter.
Third phase:
The above-mentioned rise in potential draws a current from the triggering emitter towards the main cathode. That current will be called herebelow the "triggering current".
Fourth phase:
The triggering current causes the injecting of minority carriers in the base from the main emitter through the corresponding junction, which will be called hereinafter the "main junction".
Fifth phase:
The above injection fires the main thyristor according to the conventional process. This was the aim to be achieved, the advantage of the arrangement described being that the power available for injecting at the time of the above fourth phase is much greater than that which had been applied to the trigger.
The triggering emitter is separated from the main emitter by the base, in such a way that the current flowing from the triggering emitter to the main cathode passes through the base, this being favourable, at the time of the fourth phase, to the injecting through the main junction.
The cathode is recessed in relation to the edge of the main emitter, at least on the triggering emitter side, so as not to short-circuit the main junction at the time of the fourth phase. This obliges the triggering current to cross the main junction.
The auxiliary metal spray straddles the junction between the triggering emitter and the base on the main emitter side, so that at the time of the fourth phase, the triggering current does not have to cross that junction in the reverse direction, moreover, that metal spray makes it possible to distribute the potential of the triggering emitter. The doping profile and the width of the base are chosen to give a preferential value to the resistance between the triggering emitter and the main emitter.
The function of the triggering emitter is also to increase the triggering speed of the thyristor, that is, the speed of increase of the current which may be borne by the thyristor without destruction. It is known if the current increases too rapidly, it reaches high values before the surface of the thyristor which is effectively fired is sufficient to bear these values. It is therefore necessary to accelerate the speed of increase of the fired surface. That fired surface begins at the main junction and its original speed of increase is substantially proportional to the perimeter flush with that junction, that is, to the perimeter of the outer edge of the intermediate region of the aforementioned base.
To increase that perimeter, it is a known method, in the usual case of a circular semiconductor plate, to provide that intermediate region with extensions stretching radially towards the edge of the plate and to provide these extensions with lateral branches running along arcs of circles concentric with the plate. Such extensions attain at least half the width of the main emitter, that width being measured radially.
Thus, high triggering speeds are obtained, but the area of the main emitter is greatly decreased and the same applies to the maximum current which the thyristor can bear when it is completely fired.
The aim of the present invention is to make a high triggering speed of a thyristor compatible with a high value of maximum intensity which the thyristor can bear when it is completely fired.